This invention relates generally to apparatus and methods for monitoring a subject's arterial blood pressure and, more particularly, to such apparatus and methods that monitor arterial blood pressure non-invasively by applying a pressure sensor against tissue overlying an arterial blood vessel, to partially applanate or compress the vessel.
Two well-known techniques have been used to non-invasively monitor a subject's arterial blood pressure waveform, namely, auscultation and oscillometry. Both techniques use a standard inflatable arm cuff that occludes the subject's brachial artery. The auscultatory technique determines the subject's systolic and diastolic pressures by monitoring certain Korotkoff sounds that occur as the cuff is slowly deflated. The oscillometric technique, on the other hand, determines these pressures, as well as the subject's mean pressure, by measuring actual pressure changes that occur in the cuff as the cuff is deflated. Both techniques determine pressure values only intermittently, because of the need to alternately inflate and deflate the cuff, and they cannot replicate the subject's actual blood pressure waveform. Thus, true continuous, beat-to-beat blood pressure monitoring cannot be achieved using these techniques.
Occlusive cuff instruments of the kind described briefly above generally have been effective in sensing long-term trends in a subject's blood pressure. However, such instruments generally have been ineffective in sensing short-term blood pressure variations, which are of critical importance in many medical applications, including surgery.
One technique that has been used to provide information about short-term blood pressure variations is called arterial tonometry. One device for implementing this technique includes a rigid array of miniature pressure transducers that is applied against the tissue overlying a peripheral artery, e.g., the radial artery. The transducers each directly sense the mechanical forces in the underlying subject tissue, and each is sized to cover only a fraction of the underlying artery. The array is urged against the tissue, to applanate the underlying artery and thereby cause beat-to-beat pressure variations within the artery to be coupled through the tissue to the transducers.
The rigid arterial tonometer described briefly above is subject to several drawbacks. First, its discrete transducers are relatively expensive and, because they are exposed, they are easily damaged. In addition, the array of discrete transducers generally is not anatomically compatible with the continuous contours of the subject's tissue overlying the artery being sensed. This has led to inaccuracies in the resulting transducer signals. In addition, in some cases, this incompatibility can cause tissue injury and patient discomfort. Another drawback is that such rigid arterial tonometers have failed to correct for signal artifacts that arise when the subject's arm is moved. This is a particular problem when the subject is exercising or otherwise ambulating.
Yet another drawback to the arterial tonometer described briefly above is its inability to continuously monitor and adjust the level of arterial wall compression to an optimum level of zero transmural pressure. Generally, optimization of arterial wall compression has been achieved only by periodic recalibration. This has required an interruption of the patient monitoring function, which sometimes can occur during critical periods. This drawback is perhaps the most severe factor limiting acceptance of tonometers in the clinical environment.
Another device functioning similarly to the arterial tonometer includes a housing having a closed, liquid-filled chamber with one wall of the chamber defined by a flexible diaphragm. The device is applied against a subject's skin, with the flexible diaphragm pressed against the tissue overlying a peripheral artery, e.g., the radial artery, and several electrodes located in separate compartments of the chamber sense volume changes in the compartments that result from the beat-to-beat pressure variations in the underlying artery. Although the device seeks to replicate the arterial pressure waveform, it is considered to have a relatively low gain, making it unduly susceptible to noise. Further, the device must be calibrated periodically, during which time its continuous monitoring of the subject's blood pressure waveform necessarily is interrupted.
It should, therefore, be appreciated that there is a continuing need for an apparatus, and related method, for non-invasively and continuously monitoring a subject's blood pressure, with reduced susceptibility to noise and without the need to intermittently interrupt the device's normal operation for calibration. Various embodiments of the present invention can fulfill some or all of these requirements.